Wrought nickel base superalloys

ABSTRACT

A wrought nickel base superalloy having an exacting combination of tungsten, tantalum, aluminum, nickel, boron, zirconium, carbon, manganese, silicon, chromium, cobalt, molybdenum, and titanium for minimization of deleterious phase formations and a resultant alloy capable of being hot worked and having good strength properties at high temperatures.

llnited States Patent [72] inventors Louis W. Lherbier Cuddy; Frank J.Rizzo, Pittsburgh, both of Pa. [21] Appl. No. 703,978 [22] Filed Feb. 8,1968 [45] Patented Nov. 2, 1971 [73] Assignee Cyclops CorporationSpecialty Steel Division Bridgeville, Pa.

[54] WROUGHT NICKEL BASE SUPERALLOYS 5 Claims, 3 Drawing Figs.

[52] US. Cl 75/171, 148/11.5, 148/325 [51] Int. Cl C22c 19/00 [50] Fieldof Search 75/171,

[56] References Cited UNITED STATES PATENTS 3,164,465 l/1965 Thielemann75/171 3,304,176 2/1967 Wlodek 75/171 3,322,534 5/1967 Shaw et a1 75/171Primary Examiner Richard 0. Dean Att0rney-Webb, Burden, Robinson & WebbABSTRACT: A wrought nickel base superalloy having an exactingcombination of tungsten, tantalum, aluminum, nickel, boron, zirconium,carbon, manganese, silicon, chromium, cobalt, molybdenum, and titaniumfor minimization of deleterious phase formations and a resultant alloycapable of being hot worked and having good strength properties at hightemperatures.

Stress (psi x IO' o Lherbier et ol. Alloy u Present-day Alloy ParameterT(20+ log t) x IO PATENTED Neva I971 SHEET 2 BF 2 INVENTORS. Louis WLherbier Frank J Rizzo M Mi THE IR A TLQRNE Y5 WROUGHT NICKEL BASESUPERALLOYS This invention relates to high-temperature alloys. Moreparticularly, it relates to a nickel base alloy capable of being hotworked and having an exacting combination of elements for achievement ofadequate strength, creep, ductility, and corrosion properties up to1,900 F.

Many superalloys have been developed for the aerospace industry and thelike, but the utility of such alloys has been limited by the failure ofthe alloys to possessthe desired hightemperature strength properties,while at the same-time having adequate ductility for hot working. Withthe trend-toward the developmentof engines with higher and higherthrusts, increased demands have been made on the component parts of suchequipment.

The desire to increase the strength of nickel base alloys has resultedin the continual addition of solid strengtheners such as chromium,molybdenum, and tungsten as well as other elements such as aluminum andtitanium. This has led to the transition from lower strength nickel basealloys with adequate ductility to thehigher strength castnickel basealloys with very limited ductility. Alloys which must be cast ratherthan fabricated do not have the' necessary high degree of uniformityeven when the finest precision casting procedure .is employed. Theincreased contentsof many of the elements has also led tooverallinstability of the alloy because of the precipitation of phasesdetrimental to both the strength and the ductility of the alloy. Thisinstability can occur afterheat treatment and/or after exposure forextended periods of time at elevated temperatures.

Our invention provides a nickel base alloy with acceptable strengthproperties up to 1,900 F. which is a significant improvement overcurrently employed alloys. The alloy still maintains adequate hotworkability so it can be'extruded,

forged and/or hot rollerinstead of cast. An improvement in,

oxidation resistance has also been effected. Ductility and corrosionresistance have not been adversely affected.

Even though the amount of the alloying additions in'this material hasbeen significantly increased overthe present day wrought alloys, thestability thereof has not been adversely affected. The term wrought, asused herein to define our al-1' loy, is intended to define an alloywhich can be hot worked; that is, extruded, forged and/or hot rolled.

The above results have been achieved by using an exacting combination ofelements to achieve the high strengths without sacrificing any of theother properties normally-adversely affected by increased strengthlevels. This proper balance of elements employed achieves these resultsby minimizing the formation of deleterious phases which lead-to theinstability of the alloy. The technique of avoiding undesirable phasesby calculating electron vacancy number was used to design our alloy. 7

The structure of nickel base superalloys consists primarily of gammaprime, carbide precipitates, and a gamma matrix. If

the composition of the material is not adequately controlled, unwantedphases form by nucleating on carbides and feeding on the gamma matrixfor their constituents. These unwanted phases deleteriously affect thestability and the strength of the material. They are the intermetallicphases sigma, mu, and Laves. Consequently, the composition of the matrixis of major concern in developing new nickel base superalloys. Thematrix of our alloy consists of nickel, cobalt, chromium, molybdenum,tungsten, andtantalum. The relative amounts of these alloying elementsin the matrix are determined by the other elements in the alloy such asaluminum, titanium, carbon, and boron, all of which have reacted to formprecipitating phases.

The alloys of this invention have been vacuum melted although it isbelieved that with the use of other proper melting techniques theimprovements mentioned herein would be attainable.

The carbon content should be between 0.25 percent and 0.45 percent witha preferred range of 0.30 percent to 0.40 percent. At least 0.25 percentwhich is high by present day standards is necessary in our alloy to makeit workable. Howsolution ever, extremely high carbon contents (i.e.,above 0.45 percent) are not desirable because the alloy will becomebrittle. The elements responsible for solid solution strengthening willform carbides, and the carbides will form in morphologies which areharmful to the desired properties.

sion resistance, solid solution strengthening and grain boundarystrengthening. Chromium content below 11.0 percent will not give thedesired resistance to oxidation and corrosion, and chromium in excess of17.0 percent makes the alloy difficult to hot work and the stabilitywill be s'everelyimpaired.

Cobalt is employed in the broad range from 8.0-12.0 percent (9.0-1 1.0percent preferred) for its strength properties at elevated temperatures.It also improves ductility, workability, and creep properties. Cobalt inexcess of 12.0 percent impairs oxidation andcorrosion properties.

Tantalum-is a critical element because of its solid solution hardeningand carbide formation. Amounts of 1.0-3.0 percent are desirable with apreferred range of 1.25 to 1.75 percent. Levels above 3.0 percentdetrimentally affect ductility and hot workability and can also lead tothe formation of harmful second phases.

Molybdenum andtungsten also take part in carbide formation and solidsolution-hardening. The broad range of molybdenum is.2.06.5 percent andthe preferred range is 2.5-3.5 percent. The low end of the range isdesirable because the presence of highmolybdenum can lead to theprecipitation of deleterious phases. The tungsten content ranges from4.0 to 8.0 percent with a preferred range of 5.5 to 6.5 percent.

Aluminum and titanium are also critical elements because of theircontribution to the strengthening of nickel base superalloys. The broadcomposition range for aluminum is 4.0 to

5.0 percent. An improved composition range is 4.20 to 4.80 percent andthe preferred range is 4.45 to 4.70 percent. This is a critical rangebecause below 4.0 percent aluminum the alloy will not achieve thestrength level and above 5.0 percent the alloy shows severe loss ofductility, workability, and stability. The broad composition range fortitanium is 2.2 to 3.2 percent with a preferred range of 2.8 to 3.2percent. Titanium which is also a carbide former has essentially thesame effect as the aluminumf Boron and zirconium both enhance the'creepresistance at elevated temperatures. The broad range for boron is 0.0005to 0.030 percent with a preferred range of 0.008 to 0.018 percent.Zirconium has a broad range of 0.001 to 0.25 percent and a preferredrange of 0.005 to 0.150 percent. An excess of either of these elementswill have deleterious effects on the ductility and stability of thealloy.

The base metal for the alloy is nickel. lts ability to harden byprecipitation .of secondary phases and carbides in addition to solidsolution strengthening makes it ideal for this application.

The nominal chemical analysis and the preferred ranges of the elementsofour alloy are summarized in table 1:

Cobalt 9-0010 The stress rupture properties of our alloy are given intable Molybdenum 2.00 to 6.50 2.50 to 3.50 I. Tungsten 4.0010 8.00 5.5010 5.50 Tam, LOO, 300 L75 TABLE III.-S'IRESS RUPIURE PROPERTIES $12 :28288 2'2: 2'28 5 tIest Ruptture Rupture Elongation Redurctlon em s resspcrcen 0 area Boron 0.0005 to 0.030 0.008 to 0.018 F?) (p.s.1.) (hours)1") (percent) Zirconium 0.001 to 0.250 0.005 to 0.150 H t N (58 0.: N H.f ki iff? 1 1, 500 70,000 100.1 4.1 3.0 1, 500 70,000 63. 8 3. 4 7. 51,500 70,000 177. 7 7. 7 14.11 1, 600 55, 000 52. 0 0. 0 0. 0 1,00055,000 01.0 7.5 8.5 All percents prelented herein unless otherwisespecified refer to weight percent. i g 1, 000 55,000 78. 0 2. 7 7. 01,000 55,000 81. 0 5. 0 10.11 1,0 55,000 70. 2 s. 5 11.11 1, 800 20, 08802. 0 11. 1 1 15 1,800 20,0 11 .0 0.7 2.: Trace elements such as sulfur,phosphorous, lead, etc. and 1.388 .88 residual iron are permitted asresiduals resulting from normal 800 20:000 2 melting practices. iron maybe present in amounts up to 2.00 1,388 fgdfi 3 2 percent, but it ispreferred that the iron content be kept below 1:900 000 5 1 7 1 0percent 20 1,000 15,888 28.11 20 1 2 1 1 000 14. J .5 0 .1' Some of themcchamcal properties of this new alloy comared to those currentlavailable are shown in table 11: 1 in order to com are the stress ruture ro erties of our P y P P P P A 1 alloy with the present day alloy,the Larson-Miller parameter method, which is well known to those skilledin the art, was 25, employed on our stress rupture data to provide therupture stress at 100 hours at various test temperatures. The resultsTABLE II.MECHANICAL PROPERTIES g are shown 1n table IV:

T t Ultimate 0i21, Elonga- R d n es ensie y 0 tion 0 uc 0n temp strengthstrength (percent of area 1 TABLE Iv F.) (p.s.i.) (p.s.i.) in 1")(percent) Heat No.: 4 i 1 201 000 153 200 12.2 1 .1 2. 1071000 170: 2000. 2 9. a one Hundred Hour 3. 218, 08g 172, 300 12. 3 :3; 1 206 1 15515. tr 3 Pro I 6m gggggg 3% g 12% S ess Ruptur pe ms 7... 150 1 .1 1. 10I700 1471000 0. 0 12.1 2. 175,000 100, 300 4.0 8. 1 1. 105, 200 142,00012. 0 111. 0 Ruvwrs Sims ti. 173, 500 12g, 1% 2. 7 8.1 Alloy Test Temp.(5) (P151) 7. 105,700 1 7 8.7 13.2 1. 125,400 1001 800 s. 7 1s. 0 3.155,288 187,988 18.3 40 Lherbieret 01. 1,500 71,000 1 8 11:11:17.. 122:2.2:: 0. 124, 300 104, 700 5. 0 7. 8 7 118 500 08 000 7. s 13. 4 Day 0:1, 113000 02: 100 10.0 40.1 Lherbier 0111. 1.700 35,000 Present dayalloy. 204, 000 140, 000 Present Day 1,700 27,000 400 000 000 1.551111"=1 11. 1,1100 22.000 600 921000 Present Day 1,1100 15,000 1.11mi" =1.11. 1,000 11.000 s- 2 Present Day 1,900 7,000

notch sensitivity. The present day alloy because of its exces- This datais also presented in graph form in the accompanying FIG. 1 where logstress is plotted against a rupture life term which includes both timeand temperature. The rupture life term is represented by theLarsomMiller parameter T(20+log t) where T is temperature in F. and t istime in hours.

The results of table 1V show a significant improvement in rupture stressat 100 hours for all test temperatures when comparing our alloy to thepresent day alloy. The consistency of the improvement can best be seenby referring to FIG. 1 where the relationship of stress and rupture lifeof our alloy is compared to the modern day alloy.

The chemical analysis of the experimental heats and the present dayalloy used in the compilation of mechanical properties are set forth intable V:

TABLE V.--AC'1UAL COMPOSITION (WEIGHT PERCENT) ANI) ELECTRON VACANCYNUMBER.

At elevated temperatures, there is a need for high strength propertiesand optimum ductility. Excessive ductility at elevated temperaturesresults in poor creep rates and insufficient ductility results in notchsensitivity for brittle fracture. Our strength levels, as measured byultimate tensile strength and yield strength, are significantly superiorto the present day alloy while the ductility, as measured by elongationand reduction of area, is optimized to prevent excessive creep rates orsive ductility at elevated temperatures is prone to both excessive creeprates and resultant dimensional instability.

C Cr Co Mo W Ta Ti Al Zr B Ni N Nominal composition (weight percent)0.06 15.00 15. 00 5. 25 3. 00 4. 40 0. 03 1301111100.. 0.26 11.81 11.075. 6.10 1.00 3.06 5.111 0.078 0.017 ...do.... U7

1 Present day alloy. 2 Ch. g

Oxidation resistance as determined by weight gain at measured timeintervals and elevated temperatures and corrosion properties asdetermined by sulfidation resistance are comparable to the present dayalloy.

The hot workability of the alloy is evidenced by the fact that theingots produced have been extruded, forged, or hot rolled. Theaforementioned three methods of hot workinghave also been successfullyaccomplished in various combinations thereof. Ingots have also been hotrolled directly into billets, bars and sheets.

The method of calculating an electron vacancy number (N for an alloy isknown to those skilled in the art.=rBasically, the average number ofelectron vacancies (N..) in an alloy is calculated by forming theatom'fractions of the elements involved. The alloy matrix compositionthen becomes a critical part in N calculations and all precipitatedphases are subtracted'from the total alloy composition. The type,amount, and compositions of the precipitated phases are first determined-by various empirical means also well known to those skilled in the art.

More specifically, as will be shown in the example, the alloyingelements in the material are converted to atomic percent. Followingcalculation of boride, carbides, and gamma prime phases precipitated,the residual alloying elementsare assumed to constitute the matrix. Thematrix elementamounts are scaled to 100 percent and the new matrixcomposition is then used to calculate the mean electron vacancy numberby summation.

A sample calculation of the electron vacancy number ofone of our heatsdesignated as heat 5 is illustrated next.

The Actual Procedure Employed in Calculating the Electron Vacancy Number(N,) for Heat No.5

Initially the matrix contains atoms of each element l presented by theatomic percent. The following reactions take place:

a. Loss of elements to formation of borides 0.5 o.15 o.25 o.1o)a zResidual element:

Ni 57.90 (0.30 =='57.89 Cr 13.36- (0.75 X 13.33

Left after boride.

b. Loss of elements to formation of carbides where M represents one ormore metallic elements.

MC+M C[Ni CoM0 )C] Residual Element L66 Percent Carbon available MC0.83percent Carbon Zr=0.070.07=0 Left after MC reaction .C. Loss of elementsto formation of gamma prime.

Ni (Al+Ti+0.l Cr) Residual Element Cr=l 3.33l.33=l2.00 Left after gammaprime formation The matrix now contains the atoms ofeach element whichhas not taken part in the above reactions. That leaves:

Residual Percentage of .Elcment Atoms (5b) Residuals in Matrix C 0.0 Cr12.00 33.71 Co 8.87 24.92 M0 0.0 0.0 W l.07 3.01 Ta 0.0 0.0 A1 0.0 0.0Ti 0.0 00 Zr 0.0 B 0.0 0.0 Ni 13.66 38.37

The electron vacancy number (N,.) is determined fromthe followingequation:

'FIG. ,2, and an unacceptable microstructureFlG. 3. FIG. 2 is aphotomicrograph of a-sample from heat ,5 having anelectron vacancynumber of 2.39. FIG. 3 is a photomicrograph of a sample from heat "9which has a chemical-composition outside of our broad range (see tableV) and has an electron vacancy number of 2.97. Both photomicrographs aretaken from sam- ,ples in theidentical as heat-treated state and aretaken at 4,000 magnifications.

vIn FIG. 2 the .large almost spherical particles are MC carbide. Thesmaller somewhat spherical particles precipitated at grain boundariesare M C carbides. Within the grains, numerous angular particles of gammaprime have precipitated.

From grain to grain, the orientation of the gamma -.pr,ime is changed.

In FIG. 3, several large spherical MC carbides surrounded by smalleralmost spherical particles of primary gamma prime are evident. Thegrains contain many small angular gamma prime precipitates. Theundesirable needlelike" phase present throughout the structure is of thesigma type. The presence of this undesirable phase results ininstability of the alloy and poor mechanical properties. The poormechanical properties are evident by comparing heat 9 in table II withthe properties of our alloy.

When the sigma-type needles form in the microstructure, the propertiesaredetrimentally effected because alloying elements employed for solidsolution strengthening are used to form the needle1ike" phase instead.Thus, the strength decreases as the needles form and grow. Failureoccurs because of excessive slip and the needles" act as excellentplanes upon which slip can occur. Therefore, the more sigmatype phasepresent, the greater the resultant instability of the alloy.

The difference in microstructure becomes more acute after the alloy hasreceived exposure, i.e., exposure for extended periods of time atelevated temperatures. The presence of these unwanted second phasesgreatly reduce the stability of the alloy and therefore limit the typeof use for which the alloy can be employed. Therefore, to maintain thestability of the alloy, it must meet a satisfactory N, range of values.At the same time, the composition must have the necessary high strengthmechanical and stress rupture properties.

Alloys having electron vacancy numbers below 1.9 do not possess therequisite high-temperature properties. As the electron vacancy number isincreased above 1.9, the requisite strength properties increase and thestability of the alloy remains satisfactory. The first notabletransition from stability to the presence of deleterious second phasesoccurs above an N of 2.5 in the exposed state. In the heat-treated statebefore exposure, the formation of the deleterious phases usually occursabove an electron vacancy number of 2.7. In addition, above an N of 2.5the strength and ductility properties start to diminish. However, analloy having an N number from 2.5 to 2.7 is still quite satisfactory formany applications both from the standpoint of mechanical properties andstability. However, the optimum high-temperature properties are found incompositions having an N, range from 2.3 to 2.5.

We claim:

1. A wrought nickel base alloy for use up to l,900 F. composing byweight percent 0.25 to 0.45 carbon, 0 to 2.00 manganese, 0 to 1.50silicon, 11.00 to 17.00 chromium 8.00 to 12.00 cobalt, 2.00 to 6.50molybdenum, 4.00 to 8.00 tungsten, 1.00 to 3.00 tantalum, 4.00 to 5.00aluminum, 2.20 to 3.20 titanium, 0.0005 to 0.030 boron, 0.001 to 0.250zirconium, 2.0 max. iron, and the balance nickel.

2. An alloy of the composition set forth in claim 1 characterized by anelectron vacancy number of 1.9 to 2.7.

3. A wrought nickel base alloy for use up to 1,900 F. composing byweight percent 0.30 to 0.40 carbon, 1.00 max. manganese, 1.00 max.silicon, 11.00 to 13.00 chromium, 9.00 to 11.00 cobalt, 2.50 to 3.50molybdenum, 5.50 to 6.50 tungsten, 1.25 to 1.75 tantalum, 4.45 to 4.70aluminum, 2.80 to 3.20 titanium, 0.008 to 0.018 boron, 0.005 to 0.150zirconium, 1.0 max. iron, and balance nickel.

4. An alloy of the composition set forth in claim 3 characterized alsoby an electron vacancy number in the range of 2.3 to 2.5.

5. The alloy of claim 3 containing up to 0.50 percent by weight mischmetal, the misch metal composing a mixture of rare earth elements inmetallic form.

* I l i i

2. An alloy of the composition set forth in claim 1 characterized by anelectron vacancy number of 1.9 to 2.7.
 3. A wrought nickel base alloyfor use up to 1,900* F. composing by weight percent 0.30 to 0.40 carbon,1.00 max. manganese, 1.00 max. silicon, 11.00 to 13.00 chromium, 9.00 to11.00 cobalt, 2.50 to 3.50 molybdenum, 5.50 to 6.50 tungsten, 1.25 to1.75 tantalum, 4.45 to 4.70 aluminum, 2.80 to 3.20 titanium, 0.008 to0.018 boron, 0.005 to 0.150 zirconium, 1.0 max. iron, and balancenickel.
 4. An alloy of the composition set forth in claim 3characterized also by an electron vacancy number in the range of 2.3 to2.5.
 5. The alloy of claim 3 containing up to 0.50 percent by weightmisch metal, the misch metal composing a mixture of rare earth elementsin metallic form.